Secure tags are used for a number of different purposes; a primary purpose being preventing, detecting, and/or deterring counterfeiting of an item to which the secure tags are affixed.
One type of secure tag that has recently been developed is based on small particles of a rare earth doped host, such as glass. This type of secure tag is described in U.S. patent application No. 2004/0262547, entitled “Security Labelling,” and U.S. patent application No. 2005/0143249, entitled “Security Labels which are Difficult to Counterfeit”, both of which are incorporated herein by reference.
These rare earth doped particles (hereinafter “RE particles”) can be applied to valuable items in different ways. For example, the secure tags can be incorporated in fluids which are applied (by printing, spraying, painting, or such like) to valuable items, or incorporated directly into a substrate (paper, metal, rag, plastic, or such like) of the valuable items.
In response to suitable excitation, RE particles produce a luminescence spectrum having narrow peaks because of the atomic (rather than molecular) transitions involved. Luminescence is a generic term that relates to a substance emitting optical radiation in response to excitation, and includes photoluminescence, such as fluorescence and phosphorescence.
Fluorescent materials (dyes and pigments) typically have a decay lifetime of 10−9 to 10−7 seconds (1 to 100 nanoseconds). The fluorescence disappears very quickly after excitation ceases. Thus, detecting fluorescence is typically performed simultaneously with excitation.
Phosphorescent materials (dyes and pigments) typically have a decay lifetime of 10−3 to 100 seconds. Although detecting phosphorescence can be done simultaneously with excitation, it is also possible to measure phosphorescence after the excitation is removed, thereby adding to the security of a phosphorescent secure tag.
One advantage of secure tags based on RE particles is that luminescence from these RE particles persists for a relatively long period of time after an excitation source is removed; that is, the luminescence decay time is similar to that of phosphorescent materials. This enables a luminescence detector to include a delay between excitation and detection so that background fluorescence decays prior to the luminescence from the RE particles being detected.
To enable quick and accurate validation of a secure tag, a luminescence signature is derived from the luminescence measured from that secure tag. This luminescence signature may be based on peak locations, absence of peaks, relative peak intensities, and such like. A luminescence signature is typically derived by converting a large number of data points from a luminescence spectrum into a relatively short code. This short code (the luminescence signature) enables rapid comparison with other, pre-stored luminescence signatures to facilitate validation of the secure tag.
It would be desirable to increase the security of secure tags based on RE particles to make them even more difficult to counterfeit, without making validation of the RE particles slower or more expensive.